In the manufacture of organic semiconductor devices it is often desired to create electrodes that are spaced by a distance that is as small as possible to minimize resistance and therefore to maximize the speed of the device and its capacity to carry current. It is also preferable to make the electrodes as narrow as possible to reduce their capacitance, e.g. gate overlap capacitance. Conventionally, the electrodes are formed by one of three methods as follows:
Laser ablation. In this method, a metalized plastic substrate is patterned by using a laser to remove unwanted material.
Photolithography. In this method, photo resist is spun onto a substrate and then exposed to UV radiation through a patterned mask. The photoresist is then developed to allow unwanted parts of the resist to be removed, thereby uncovering an underlying metal layer. The uncovered parts of the metal are then removed by etching. In a variation of this method, there is no underlying metal layer; instead metal is evaporated onto the substrate in areas where it has been exposed by removal of the photo resist. Unwanted conductive material that is deposited over the photo resist is then removed by a solvent.
Conductive “Ink”. In this method, a conductive material is printed directly onto a substrate using a process similar to that used in a conventional inkjet printer.
The electrically-conductive material can be metallic or can be a conductive organic material, for example, poly(3,4-ethylenedioxythiopene) PEDOT.
In another variation, a resist is printed by a jetting process onto a metalized substrate before etching. These methods all have shortcomings. Laser ablation gives good resolution, but is not conductive to large-area or high volume production, and requires specialized and expensive material. Photolithography gives good resolution but is slow, can not be used over a large area, and requires the step of removing the resist before application of the semiconductor. Printing with metallic inks results in an undesirably rough surface and is restricted to use with metals that can be formed into an ink and which, for that reason, do not have ideal electrical properties. For example, silver can be formed into suitable ink, but is a poor injector of charge. Electrically-conductive organic materials are available that are good as injectors of charge, but they are not good conductors of electricity.
This invention arose with the aim of overcoming some or all of the above-mentioned problems.